1 6 MEMOIIfS OF THE NATIONAL ACADEMY OF SCIENCES. 



mentation, i. c, the primaij' factors coucerued in tlieir evolution. Weismann in Ids earlier work 

 repeatedly a.sserts that the.se changes are due to the direct action of external conditions together 

 with natural selection. Within a few years past many naturalists have returned to a more profound 

 study of the causes of variation along some of the lines vaguely pointed out by Lamarck.' It is 

 noteworthy that Darwin changed his views somewhat in his Variation of Animals and Plants under 

 Domestication, and laid more stress on the influence of the surroundings than in his Origin of 

 Species. 



Neither Weismann nor other authors, however, .so far as we know, have formally discussed 

 the jirobable mode of origin of humps, horns, tubercles, si)ines, and such outgrowths in larvie. 

 They are so marked and so manifold in their variations in form, and so manifestly related, and in 

 fact have so evidently been directly developed by adaptation to changes in the habits of the 

 Notodontiau caterpillars and tree-feeding larvie in general tliat this group affords favorable material 

 for a study of the general problem. 



Spines and prickles in animals, like those of plants, serve to protect the organism from external 

 attack, and also to strengthen the shell or skin; they are adaptive structures, and have evidently 

 arisen in response to external stimuli, either those of a general or of a cosmical nature, or those 

 resulting from tlic attacks of animals. It is almost an axiomatic truth that a change of habit 

 in the organism precedes or induces a change of structure. 



What has caused the enlargement and specialization of certain of the piliferous warts ? As 

 remarked by Sir James Paget, " Constant extrapressure on a part always appears to produce 

 atrophy and absorption; occasional pressure may, and usually does, jn-odnce hypertrophy and 

 thickening. All the thickenings of the cuticle are the consequences of occasional pressure, as the 

 pressure of shoes in occasional walking, of tools occasionally used with the hand, and the like, 

 for it seems a necessary condition for hypertrophy, in most parts, that they should enjoy intervals 

 in which their nutrition may go on actively." (See Lectures on Surgical Pathology, I, p. 89, quoted 

 by Henslow, who remarks in his suggestive work, "The origin of floral structures through insect 

 and other agencies."' that "the reader will perceive the significance of this passage when recalling 

 the fact that insects' visits are intermittent."-) 



It is now assumed by some naturalists that the thorns, spines, and prickles of cacti and other 

 plants growing in desert or dry and sterile places are due either to defective nutrition or to " ebbing 

 vitality" (Geddes), or by others, as Mr. Wallace, to the stimulus resulting iiom the occasional 

 attacks or visits of animals, especially mammals. It should be borne in mind that the great deserts 

 of the globe are of quite recent formation, being the result of the desiccation of interior areas of 

 the continents, late in the Quaternary epoch, succeeding the time of liver terraces. Owing to this 



' Herbert Spencer says : "The direct action of the medium was the primordial factor of organic evolution " (see 

 The Factor of Organic Evolution. 1>*86). Claude Bernard wrote: " The conditions of life are neither in the organism, 

 nor in its external surroundings, I>ut in both at ouce" .'([UOted from J. A. Thompson's Synthetic Summary of the 

 lutluence of the Environment upon the Organism, Proc. Eoy. I'hys. Soc, is, 1888). Sachs remarks: "A far greater 

 portion of the phenomena of life are [is] called forth by external iiitlueuces than one formerly ventured to assume" 

 (Phys. of Plants, 1887, IPl, English translation). Semper claims '•' that of all the properties of the animal organism, 

 variability is that which may first and most easily be traced by exact iuvestigatiou to its efficient causes" (Animal 

 Life, etc., preface, vi). "External conditions can exert not only a very powerful selective influence, but a transform- 

 ing one as well, although it must bo the more limited of the two'' (lb., 37). "No power which is able to act only as 

 a selective, and not as a transforming, iulliieuce can ever be exclusively put forward as the proper efficient cause — 

 g'tiisa efficiens — of any phenomenon (lb., 404). 



-Henslow also adds that "atrophy by pressure and absorption is seen in the growth of embryos, while the 

 constant pressure of a ligature arrests all growth at the constricted place. On the other hand, it would seem to be 

 the persistent contact which causes a climber to thicken." 



It mayhere be noted that the results of the hypertrophy and overgrowth of the two consolidated tergites of 

 the second anteunal and mandibular segments of the Decapod Crustacea, by which the carapace has been produced, 

 h.as resulted in a coustaut pressure on the dorsal arches of the succeeding five cephalic and five thoracic segments, 

 until as a result we have an atrophy of the dorsal arches of as many as ten segments, these being covered by the 

 carapace. Audouiu early in this century enunciated the law that in articulated animals one part was built up at 

 the expense of adjoining portions or organs, and this is beautifully exemplified by the changes in the development 

 of the carapace of the embryo and larval Decapod Crustacea, and also in insects. For example, note the change in 

 form and partial atrophy of the two hinder thoracic somites of some beetles, as compared with the large prothorax, 

 due probably to the more or less continual pressure exerted by the folded elytra and wings. 



